(Reuters) - NFL linebacker Junior Seau, who committed suicide last year, had a debilitating brain disease, likely from 20 years of blows to the head, ABC News and ESPN reported on Thursday, citing researchers and his family as sources.

Seau, 43, died in May after shooting himself in the chest, one of a handful of former players to have committed suicide after their retirement. He had played for the San Diego Chargers and had a 20-year career in the National Football League.

A study of Seau's brain by a team of independent researchers found that he had suffered from chronic traumatic encephalopathy, or CTE, the report said.

CTE can only be diagnosed after death.

Tissue from his brain was sent to the National Institutes of Health for analysis in July, at the request of Seau's family, amid growing concerns over the long-term effects of football-related head injuries.

The NIH was not immediately available for comment.

"What was found in Junior Seau's brain were cellular changes consistent with CTE," Dr. Russell Lonser, the lead researcher on the case, told ABC and ESPN. Lonser is chairman of the Department of Neurological Surgery at Ohio State University, and led the study of Seau's brain while he was at NIH.

Patients with CTE may display symptoms "such as impulsivity, forgetfulness, depression, sometimes suicidal ideation," Lonser said, according to the report.

It was unclear what effect the study of Seau's brain would have on a class action suit brought by several thousand former NFL players who claim the league has not done enough to protect players from brain injuries.

The NFL said the result of the examination of Seau's brain underscored "the recognized need for additional research to accelerate a fuller understanding of CTE."

"The NFL clubs have already committed a $30 million research grant to the NIH, and we look forward to making decisions soon with the NFL Players Association on the investment of $100 million for medical research that is committed in the Collective Bargaining Agreement," the NFL said in a statement. "We have work to do, and we're doing it."

Just weeks before Seau shot himself, former Atlanta Falcons safety Ray Easterling committed suicide and family members described a long descent into dementia following his retirement from the NFL. An autopsy revealed indications of CTE.

In February 2011, four-time Pro Bowl safety Dave Duerson, who played most of his career with the Chicago Bears, shot himself in the chest. In a suicide note, he donated his brain for study, and it was found to exhibit signs of CTE.